This invention relates to a catalytic technique for upgrading olefin streams to gasoline streams rich in aromatics. In particular, it provides a continuous process for oligomerizing and aromatizing a feedstock containing light C.sub.4 - olefins to produce C.sub.5 + hydrocarbons rich in C.sub.6 -C.sub.10 aromatics, such as benzene, toluene, xylenes, tri- and tetramethylbenzenes together with hydrogen and fuel gas.
Developments in zeolite catalysis and hydrocarbon conversion processes have created interest in utilizing olefinic feedstocks for producing C.sub.5 + gasoline, distillate and lubricants. In addition to the basic chemical reactions promoted by medium-pore zeolite catalysts, a number of discoveries have contributed to the development of new industrial processes. These are safe, environmentally acceptable processes for utilizing feedstocks that contain olefins. Conversion of C.sub.2 -C.sub.4 alkenes and alkanes to produce aromatics-rich liquid hydrocarbon products were found by Cattanach (U.S. Pat. No. 3,760,024) and Yan et al. (U.S. Pat. No. 3,845,150) to be effective in the presence of a medium-pore zeolite catalysts. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclosed conversion of C.sub.2 -C.sub.5 olefins, alone or in admixture with paraffinic components, into higher hydrocarbons over crystalline zeolites having controlled acidity. Garwood et al. have also contributed to the understanding of catalytic olefin upgrading techniques and improved processes as in U.S. Pat. Nos. 4,150,062; 4,211,640 and 4,227,992. The above-identified disclosures are incorporated by reference as if set forth at length herein.
Conversion of olefins is effective in the presence of medium-pore zeolite catalysts at moderately elevated temperatures and pressures. The conversation products are sought as liquid fuels, especially the C.sub.5 + aliphatic and aromatic hydrocarbons. Product distribution for liquid hydrocarbons can be varied by controlling process conditions, such as temperature, pressure and space velocity. Aromatic gasoline (C.sub.5 -C.sub.10) is readily formed at elevated temperature (e.g., about 420.degree. to 650.degree. C.) and moderate pressure from ambient to about 5500 kPa, preferably about 200 to 2900 kPa. Olefinic gasoline can also be produced and may be recovered as a product or may be further upgraded to aromatic gasoline in a high severity reactor system. Alternatively, the olefinic gasoline may be charged to a low severity, high pressure reactor system for further conversion to heavier distillate range products. Operating details for typical "MOGD" oligomerization units are disclosed in U.S. Pat. Nos. 4,150,062 to Garwood et al.; 4,456,779 and 4,497,968 to Owen et al., as well as 4,433,185 to Tabak, which patents are incorporated herein by reference.
U.S. Pat. No. 3,827,968 to Givens et al. teaches a two-step process for aromatization of aliphatics. The first step comprises olefin oligomerization in the presence of a medium-pore zeolite catalyst. Oligomerized liquid product is then separated from oligomerization reactor effluent and charged to an aromatization stage. Both stages are operated in the absence of added hydrogen. The '968 patent expressly states that the second stage aromatization process is made considerably more efficient by having its feed limited to the liquid phase product of the first stage oligomerization.
With the ever-increasing demand for high octane unleaded gasoline, it would be advantageous to provide a two-stage aliphatics upgrading process which yields an increased volume of gasoline at the expense of less valuable C.sub.4 - aliphatics.